Transporters
Transporter proteins regulate many different functions of a cell, including cell proliferation, differentiation, and signaling processes, by regulating the flow of molecules such as ions and macromolecules, into and out of cells. Transporters are found in the plasma membranes of virtually every cell in eukaryotic organisms. Transporters mediate a variety of cellular functions including regulation of membrane potentials and absorption and secretion of molecules and ion across cell membranes. When present in intracellular membranes of the Golgi apparatus and endocytic vesicles, transporters, such as chloride channels, also regulate organelle pH. For a review, see Greger, R. (1988) Annu. Rev. Physiol. 50:111-122.
Transporters are generally classified by structure and the type of mode of action. In addition, transporters are sometimes classified by the molecule type that is transported, for example, sugar transporters, chlorine channels, potassium channels, etc. There may be many classes of channels for transporting a single type of molecule (a detailed review of channel types can be found at Alexander, S. P. H. and J. A. Peters: Receptor and transporter nomenclature supplement. Trends Pharmacol. Sci., Elsevier, pp. 65-68 (1997) and http://www-biology.ucsd.edu/˜msaier/transport/titlepage2.html.
The following general classification scheme is known in the art and is followed in the present discoveries.
Channel-type transporters. Transmembrane channel proteins of this class are ubiquitously found in the membranes of all types of organisms from bacteria to higher eukaryotes. Transport systems of this type catalyze facilitated diffusion (by an energy-independent process) by passage through a transmembrane aqueous pore or channel without evidence for a carrier-mediated mechanism. These channel proteins usually consist largely of a-helical spanners, although b-strands may also be present and may even comprise the channel. However, outer membrane porin-type channel proteins are excluded from this class and are instead included in class 9.
Carrier-type transporters. Transport systems are included in this class if they utilize a carrier-mediated process to catalyze uniport (a single species is transported by facilitated diffusion), antiport (two or more species are transported in opposite directions in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy) and/or symport (two or more species are transported together in the same direction in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy).
Pyrophosphate bond hydrolysis-driven active transporters. Transport systems are included in this class if they hydrolyze pyrophosphate or the terminal pyrophosphate bond in ATP or another nucleoside triphosphate to drive the active uptake and/or extrusion of a solute or solutes. The transport protein may or may not be transiently phosphorylated, but the substrate is not phosphorylated.
PEP-dependent, phosphoryl transfer-driven group translocators. Transport systems of the bacterial phosphoenolpyruvate:sugar phosphotransferase system are included in this class. The product of the reaction, derived from extracellular sugar, is a cytoplasmic sugar-phosphate.
Decarboxylation-driven active transporters. Transport systems that drive solute (e.g., ion) uptake or extrusion by decarboxylation of a cytoplasmic substrate are included in this class.
Oxidoreduction-driven active transporters. Transport systems that drive transport of a solute (e.g., an ion) energized by the flow of electrons from a reduced substrate to an oxidized substrate are included in this class.
Light-driven active transporters. Transport systems that utilize light energy to drive transport of a solute (e.g., an ion) are included in this class.
Mechanically-driven active transporters. Transport systems are included in this class if they drive movement of a cell or organelle by allowing the flow of ions (or other solutes) through the membrane down their electrochemical gradients.
Outer-membrane porins (of b-structure). These proteins form transmembrane pores or channels that usually allow the energy independent passage of solutes across a membrane. The transmembrane portions of these proteins consist exclusively of b-strands that form a b-barrel. These porin-type proteins are found in the outer membranes of Gram-negative bacteria, mitochondria and eukaryotic plastids.
Methyltransferase-driven active transporters. A single characterized protein currently falls into this category, the Na+-transporting methyltetrahydromethanopterin:coenzyme M methyltransferase.
Non-ribosome-synthesized channel-forming peptides or peptide-like molecules. These molecules, usually chains of L- and D-amino acids as well as other small molecular building blocks such as lactate, form oligomeric transmembrane ion channels. Voltage may induce channel formation by promoting assembly of the transmembrane channel. These peptides are often made by bacteria and fungi as agents of biological warfare.
Non-Proteinaceous Transport Complexes. Ion conducting substances in biological membranes that do not consist of or are not derived from proteins or peptides fall into this category.
Functionally characterized transporters for which sequence data are lacking. Transporters of particular physiological significance will be included in this category even though a family assignment cannot be made.
Putative transporters in which no family member is an established transporter. Putative transport protein families are grouped under this number and will either be classified elsewhere when the transport function of a member becomes established, or will be eliminated from the TC classification system if the proposed transport function is disproven. These families include a member or members for which a transport function has been suggested, but evidence for such a function is not yet compelling.
Auxiliary transport proteins. Proteins that in some way facilitate transport across one or more biological membranes but do not themselves participate directly in transport are included in this class. These proteins always function in conjunction with one or more transport proteins. They may provide a function connected with energy coupling to transport, play a structural role in complex formation or serve a regulatory function.
Transporters of unknown classification. Transport protein families of unknown classification are grouped under this number and will be classified elsewhere when the transport process and energy coupling mechanism are characterized. These families include at least one member for which a transport function has been established, but either the mode of transport or the energy coupling mechanism is not known.
Ion Channels
An important type of transporter is the ion channel. Ion channels regulate many different cell proliferation, differentiation, and signaling processes by regulating the flow of ions into and out of cells. Ion channels are found in the plasma membranes of virtually every cell in eukaryotic organisms. Ion channels mediate a variety of cellular functions including regulation of membrane potentials and absorption and secretion of ion across epithelial membranes. When present in intracellular membranes of the Golgi apparatus and endocytic vesicles, ion channels, such as chloride channels, also regulate organelle pH. For a review, see Greger, R. (1988) Annu. Rev. Physiol. 50:111-122.
Ion channels are generally classified by structure and the type of mode of action. For example, extracellular ligand gated channels (ELGs) are comprised of five polypeptide subunits, with each subunit having 4 membrane spanning domains, and are activated by the binding of an extracellular ligand to the channel. In addition, channels are sometimes classified by the ion type that is transported, for example, chlorine channels, potassium channels, etc. There may be many classes of channels for transporting a single type of ion (a detailed review of channel types can be found at Alexander, S. P. H. and J. A. Peters (1997). Receptor and ion channel nomenclature supplement. Trends Pharmacol. Sci., Elsevier, pp. 65-68 and http://www-biology.ucsd.edu/˜msaier/transport/toc.html.
There are many types of ion channels based on structure. For example, many ion channels fall within one of the following groups: extracellular ligand-gated channels (ELG), intracellular ligand-gated channels (ILG), inward rectifying channels (INR), intercellular (gap junction) channels, and voltage gated channels (VIC). There are additionally recognized other channel families based on ion-type transported, cellular location and drug sensitivity. Detailed information on each of these, their activity, ligand type, ion type, disease association, drugability, and other information pertinent to the present invention, is well known in the art.
Extracellular ligand-gated channels, ELGs, are generally comprised of five polypeptide subunits, Unwin, N. (1993), Cell 72: 31-41; Unwin, N. (1995), Nature 373: 37-43; Hucho, F., et al., (1996) J. Neurochem. 66: 1781-1792; Hucho, F., et al., (1996) Eur. J. Biochem. 239: 539-557; Alexander, S. P. H. and J. A. Peters (1997), Trends Pharmacol. Sci., Elsevier, pp. 4-6; 36-40; 42-44; and Xue, H. (1998) J. Mol. Evol. 47: 323-333. Each subunit has 4 membrane spanning regions: this serves as a means of identifying other members of the ELG family of proteins. ELG bind a ligand and in response modulate the flow of ions. Examples of ELG include most members of the neurotransmitter-receptor family of proteins, e.g., GABAI receptors. Other members of this family of ion channels include glycine receptors, ryandyne receptors, and ligand gated calcium channels.
The Voltage-gated Ion Channel (VIC) Superfamily
Proteins of the VIC family are ion-selective channel proteins found in a wide range of bacteria, archaea and eukaryotes Hille, B. (1992), Chapter 9: Structure of channel proteins; Chapter 20: Evolution and diversity. In: Ionic Channels of Excitable Membranes, 2nd Ed., Sinaur Assoc. Inc., Pubs., Sunderland, Mass.; Sigworth, F. J. (1993), Quart. Rev. Biophys. 27: 1-40; Salkoff, L. and T. Jegla (1995), Neuron 15: 489-492; Alexander, S. P. H. et al., (1997), Trends Pharmacol. Sci., Elsevier, pp. 76-84; Jan, L. Y. et al., (1997), Annu. Rev. Neurosci. 20: 91-123; Doyle, D. A, et al., (1998) Science 280: 69-77; Terlau, H. and W. Stühmer (1998), Naturwissenschaften 85: 437-444. They are often homo- or heterooligomeric structures with several dissimilar subunits (e.g., a1-a2-d-b Ca2+ channels, ab1b2 Na+ channels or (a)4-b K+ channels), but the channel and the primary receptor is usually associated with the a (or a1) subunit. Functionally characterized members are specific for K+, Na+ or Ca2+. The K+ channels usually consist of homotetrameric structures with each a-subunit possessing six transmembrane spanners (TMSs). The a1 and a subunits of the Ca2+ and Na+ channels, respectively, are about four times as large and possess 4 units, each with 6 TMSs separated by a hydrophilic loop, for a total of 24 TMSs. These large channel proteins form heterotetra-unit structures equivalent to the homotetrameric structures of most K+ channels. All four units of the Ca2+ and Na+ channels are homologous to the single unit in the homotetrameric K+ channels. Ion flux via the eukaryotic channels is generally controlled by the transmembrane electrical potential (hence the designation, voltage-sensitive) although some are controlled by ligand or receptor binding.
Several putative K+-selective channel proteins of the VIC family have been identified in prokaryotes. The structure of one of them, the KcsA K+ channel of Streptomyces lividans, has been solved to 3.2 Å resolution. The protein possesses four identical subunits, each with two transmembrane helices, arranged in the shape of an inverted teepee or cone. The cone cradles the “selectivity filter” P domain in its outer end. The narrow selectivity filter is only 12 Å long, whereas the remainder of the channel is wider and lined with hydrophobic residues. A large water-filled cavity and helix dipoles stabilize K+ in the pore. The selectivity filter has two bound K+ ions about 7.5 Å apart from each other. Ion conduction is proposed to result from a balance of electrostatic attractive and repulsive forces.
In eukaryotes, each VIC family channel type has several subtypes based on pharmacological and electrophysiological data. Thus, there are five types of Ca2+ channels (L, N, P, Q and T). There are at least ten types of K+ channels, each responding in different ways to different stimuli: voltage-sensitive [Ka, Kv, Kvr, Kvs and Ksr], Ca2+-sensitive [BKCa, IKCa and SKCa] and receptor-coupled [KM and KACh]. There are at least six types of Na+ channels (I, II, III, μ1, H1 and PN3). Tetrameric channels from both prokaryotic and eukaryotic organisms are known in which each a-subunit possesses 2 TMSs rather than 6, and these two TMSs are homologous to TMSs 5 and 6 of the six TMS unit found in the voltage-sensitive channel proteins. KcsA of S. lividans is an example of such a 2 TMS channel protein. These channels may include the KNa (Na+-activated) and KVol (cell volume-sensitive) K+ channels, as well as distantly related channels such as the Tok1 K+ channel of yeast, the TWIK-1 inward rectifier K+ channel of the mouse and the TREK-1 K+ channel of the mouse. Because of insufficient sequence similarity with proteins of the VIC family, inward rectifier K+ IRK channels (ATP-regulated; G-protein-activated) which possess a P domain and two flanking TMSs are placed in a distinct family. However, substantial sequence similarity in the P region suggests that they are homologous. The b, g and d subunits of VIC family members, when present, frequently play regulatory roles in channel activation/deactivation.
The Epithelial Na+ Channel (ENaC) Family
The ENaC family consists of over twenty-four sequenced proteins (Canessa, C. M., et al., (1994), Nature 367: 463-467, Le, T. and M. H. Saier, Jr. (1996), Mol. Membr. Biol. 13: 149-157; Garty, H. and L. G. Palmer (1997), Physiol. Rev. 77: 359-396; Waldmann, R., et al., (1997), Nature 386: 173-177; Darboux, I., et al., (1998), J. Biol. Chem. 273: 9424-9429; Firsov, D., et al., (1998), EMBO J. 17: 344-352; Horisberger, J.-D. (1998). Curr. Opin. Struc. Biol. 10: 443-449). All are from animals with no recognizable homologues in other eukaryotes or bacteria. The vertebrate ENaC proteins from epithelial cells cluster tightly together on the phylogenetic tree: voltage-insensitive ENaC homologues are also found in the brain. Eleven sequenced C. elegans proteins, including the degenerins, are distantly related to the vertebrate proteins as well as to each other. At least some of these proteins form part of a mechano-transducing complex for touch sensitivity. The homologous Helix aspersa (FMRF-amide)-activated Na+ channel is the first peptide neurotransmitter-gated ionotropic receptor to be sequenced.
Protein members of this family all exhibit the same apparent topology, each with N- and C-termini on the inside of the cell, two amphipathic transmembrane spanning segments, and a large extracellular loop. The extracellular domains contain numerous highly conserved cysteine residues. They are proposed to serve a receptor function.
Mammalian ENaC is important for the maintenance of Na+ balance and the regulation of blood pressure. Three homologous ENaC subunits, alpha, beta, and gamma, have been shown to assemble to form the highly Na+-selective channel. The stoichiometry of the three subunits is alpha2, beta1, gamma1 in a heterotetrameric architecture.
The Glutamate-gated Ion Channel (GIC) Family of Neurotransmitter Receptors
Members of the GIC family are heteropentameric complexes in which each of the 5 subunits is of 800-1000 amino acyl residues in length (Nakanishi, N., et al, (1990), Neuron 5: 569-581; Unwin, N. (1993), Cell 72: 31-41; Alexander, S. P. H. and J. A. Peters (1997) Trends Pharmacol. Sci., Elsevier, pp. 36-40). These subunits may span the membrane three or five times as putative a-helices with the N-termini (the glutamate-binding domains) localized extracellularly and the C-termini localized cytoplasmically. They may be distantly related to the ligand-gated ion channels, and if so, they may possess substantial b-structure in their transmembrane regions. However, homology between these two families cannot be established on the basis of sequence comparisons alone. The subunits fall into six subfamilies: a, b, g, d, e and z.
The GIC channels are divided into three types: (1) a-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-, (2) kainate- and (3) N-methyl-D-aspartate (NMDA)-selective glutamate receptors. Subunits of the AMPA and kainate classes exhibit 35-40% identity with each other while subunits of the NMDA receptors exhibit 22-24% identity with the former subunits. They possess large N-terminal, extracellular glutamate-binding domains that are homologous to the periplasmic glutamine and glutamate receptors of ABC-type uptake permeases of Gram-negative bacteria. All known members of the GIC family are from animals. The different channel (receptor) types exhibit distinct ion selectivities and conductance properties. The NMDA-selective large conductance channels are highly permeable to monovalent cations and Ca2+. The AMPA-and kainate-selective ion channels are permeable primarily to monovalent cations with only low permeability to Ca2+.
The Chloride Channel (ClC) Family
The ClC family is a large family consisting of dozens of sequenced proteins derived from Gram-negative and Gram-positive bacteria, cyanobacteria, archaea, yeast, plants and animals (Steinmeyer, K., et al., (1991), Nature 354: 301-304; Uchida, S., et al., (1993), J. Biol. Chem. 268: 3821-3824; Huang, M. -E., et al., (1994), J. Mol. Biol. 242: 595-598; Kawasaki, M., et al, (1994), Neuron 12: 597-604; Fisher, W. E., et al., (1995), Genomics. 29:598-606; and Foskett, J. K. (1998), Annu. Rev. Physiol. 60: 689-717). These proteins are essentially ubiquitous, although they are not encoded within genomes of Haemophilus influenzae, Mycoplasma genitalium, and Mycoplasma pneumoniae. Sequenced proteins vary in size from 395 amino acyl residues (M. jannaschii) to 988 residues (man). Several organisms contain multiple ClC family paralogues. For example, Synechocystis has two paralogues, one of 451 residues in length and the other of 899 residues. Arabidopsis thaliana has at least four sequenced paralogues, (775-792 residues), humans also have at least five paralogues (820-988 residues), and C. elegans also has at least five (810-950 residues). There are nine known members in mammals, and mutations in three of the corresponding genes cause human diseases. E. coli, Methanococcus jannaschii and Saccharomyces cerevisiae only have one ClC family member each. With the exception of the larger Synechocystis paralogue, all bacterial proteins are small (395-492 residues) while all eukaryotic proteins are larger (687-988 residues). These proteins exhibit 10-12 putative transmembrane a-helical spanners (TMSs) and appear to be present in the membrane as homodimers. While one member of the family, Torpedo ClC-O, has been reported to have two channels, one per subunit, others are believed to have just one.
All functionally characterized members of the ClC family transport chloride, some in a voltage-regulated process. These channels serve a variety of physiological functions (cell volume regulation; membrane potential stabilization; signal transduction; transepithelial transport, etc.). Different homologues in humans exhibit differing anion selectivities, i.e., ClC4 and ClC5 share a NO3−>Cl−>Br−>I− conductance sequence, while ClC3 has an I−>Cl− selectivity. The ClC4 and ClC5 channels and others exhibit outward rectifying currents with currents only at voltages more positive than +20 mV.
Animal Inward Rectifier K+ Channel (IRK-C) Family
IRK channels possess the “minimal channel-forming structure” with only a P domain, characteristic of the channel proteins of the VIC family, and two flanking transmembrane spanners (Shuck, M. E., et al., (1994), J. Biol. Chem. 269: 24261-24270; Ashen, M. D., et al., (1995), Am. J. Physiol. 268: H506-H511; Salkoff, L. and T. Jegla (1995), Neuron 15: 489-492; Aguilar-Bryan, L., et al., (1998), Physiol. Rev. 78: 227-245; Ruknudin, A., et al., (1998), J. Biol. Chem. 273: 14165-14171). They may exist in the membrane as homo- or heterooligomers. They have a greater tendency to let K+ flow into the cell than out. Voltage-dependence may be regulated by external K+, by internal Mg2+, by internal ATP and/or by G-proteins. The P domains of IRK channels exhibit limited sequence similarity to those of the VIC family, but this sequence similarity is insufficient to establish homology. Inward rectifiers play a role in setting cellular membrane potentials, and the closing of these channels upon depolarization permits the occurrence of long duration action potentials with a plateau phase. Inward rectifiers lack the intrinsic voltage sensing helices found in VIC family channels. In a few cases, those of Kir1.1a and Kir6.2, for example, direct interaction with a member of the ABC superfamily has been proposed to confer unique functional and regulatory properties to the heteromeric complex, including sensitivity to ATP. The SUR1 sulfonylurea receptor (spQ09428) is the ABC protein that regulates the Kir6.2 channel in response to ATP, and CFTR may regulate Kir1.1a. Mutations in SUR1 are the cause of familial persistent hyperinsulinemic hypoglycemia in infancy (PHHI), an autosomal recessive disorder characterized by unregulated insulin secretion in the pancreas.
ATP-gated Cation Channel (ACC) Family
Members of the ACC family (also called P2X receptors) respond to ATP, a functional neurotransmitter released by exocytosis from many types of neurons (North, R. A. (1996), Curr. Opin. Cell Biol. 8: 474-483; Soto, F., M. Garcia-Guzman and W. Stühmer (1997), J. Membr. Biol. 160: 91-100). They have been placed into seven groups (P2X1-P2X7) based on their pharmacological properties. These channels, which function at neuron-neuron and neuron-smooth muscle junctions, may play roles in the control of blood pressure and pain sensation. They may also function in lymphocyte and platelet physiology. They are found only in animals.
The proteins of the ACC family are quite similar in sequence (>35% identity), but they possess 380-1000 amino acyl residues per subunit with variability in length localized primarily to the C-terminal domains. They possess two transmembrane spanners, one about 30-50 residues from their N-termini, the other near residues 320-340. The extracellular receptor domains between these two spanners (of about 270 residues) are well conserved with numerous conserved glycyl and cysteyl residues. The hydrophilic C-termini vary in length from 25 to 240 residues. They resemble the topologically similar epithelial Na+ channel (ENaC) proteins in possessing (a) N- and C-termini localized intracellularly, (b) two putative transmembrane spanners, (c) a large extracellular loop domain, and (d) many conserved extracellular cysteyl residues. ACC family members are, however, not demonstrably homologous with them. ACC channels are probably hetero- or homomultimers and transport small monovalent cations (Me+). Some also transport Ca2+; a few also transport small metabolites.
The Ryanodine-Inositol 1,4,5-triphosphate Receptor Ca2+ Channel (RIR-CaC) Family
Ryanodine (Ry)-sensitive and inositol 1,4,5-triphosphate (IP3)-sensitive Ca2+-release channels function in the release of Ca2+ from intracellular storage sites in animal cells and thereby regulate various Ca2+-dependent physiological processes (Hasan, G. et al., (1992) Development 116: 967-975; Michikawa, T., et al., (1994), J. Biol. Chem. 269: 9184-9189; Tunwell, R. E. A., (1996), Biochem. J. 318: 477-487; Lee, A. G. (1996) Biomembranes, Vol. 6, Transmembrane Receptors and Channels (A. G. Lee, ed.), JAI Press, Denver, Colo., pp 291-326; Mikoshiba, K., et al., (1996) J. Biochem. Biomem. 6: 273-289). Ry receptors occur primarily in muscle cell sarcoplasmic reticular (SR) membranes, and IP3 receptors occur primarily in brain cell endoplasmic reticular (ER) membranes where they effect release of Ca2+ into the cytoplasm upon activation (opening) of the channel.
The Ry receptors are activated as a result of the activity of dihydropyridine-sensitive Ca2+ channels. The latter are members of the voltage-sensitive ion channel (VIC) family. Dihydropyridine-sensitive channels are present in the T-tubular systems of muscle tissues.
Ry receptors are homotetrameric complexes with each subunit exhibiting a molecular size of over 500,000 daltons (about 5,000 amino acyl residues). They possess C-terminal domains with six putative transmembrane a -helical spanners (TMSs). Putative pore-forming sequences occur between the fifth and sixth TMSs as suggested for members of the VIC family. The large N-terminal hydrophilic domains and the small C-terminal hydrophilic domains are localized to the cytoplasm. Low resolution 3-dimensional structural data are available. Mammals possess at least three isoforms that probably arose by gene duplication and divergence before divergence of the mammalian species. Homologues are present in humans and Caenorabditis elegans. 
IP3 receptors resemble Ry receptors in many respects. (1) They are homotetrameric complexes with each subunit exhibiting a molecular size of over 300,000 daltons (about 2,700 amino acyl residues). (2) They possess C-terminal channel domains that are homologous to those of the Ry receptors. (3) The channel domains possess six putative TMSs and a putative channel lining region between TMSs 5 and 6. (4) Both the large N-terminal domains and the smaller C-terminal tails face the cytoplasm. (5) They possess covalently linked carbohydrate on extracytoplasmic loops of the channel domains. (6) They have three currently recognized isoforms (types 1, 2, and 3) in mammals which are subject to differential regulation and have different tissue distributions.
IP3 receptors possess three domains: N-terminal IP3-binding domains, central coupling or regulatory domains and C-terminal channel domains. Channels are activated by IP3 binding, and like the Ry receptors, the activities of the IP3 receptor channels are regulated by phosphorylation of the regulatory domains, catalyzed by various protein kinases. They predominate in the endoplasmic reticular membranes of various cell types in the brain but have also been found in the plasma membranes of some nerve cells derived from a variety of tissues.
The channel domains of the Ry and IP3 receptors comprise a coherent family that in spite of apparent structural similarities, do not show appreciable sequence similarity of the proteins of the VIC family. The Ry receptors and the IP3 receptors cluster separately on the RIR-CaC family tree. They both have homologues in Drosophila. Based on the phylogenetic tree for the family, the family probably evolved in the following sequence: (1) A gene duplication event occurred that gave rise to Ry and IP3 receptors in invertebrates. (2) Vertebrates evolved from invertebrates. (3) The three isoforms of each receptor arose as a result of two distinct gene duplication events. (4) These isoforms were transmitted to mammals before divergence of the mammalian species.
The Organellar Chloride Channel (O-ClC) Family
Proteins of the O-ClC family are voltage-sensitive chloride channels found in intracellular membranes but not the plasma membranes of animal cells (Landry, D, et al., (1993), J. Biol. Chem. 268: 14948-14955; Valenzuela, Set al., (1997), J. Biol. Chem. 272: 12575-12582; and Duncan, R. R., et al., (1997), J. Biol. Chem. 272: 23880-23886).
They are found in human nuclear membranes, and the bovine protein targets to the microsomes, but not the plasma membrane, when expressed in Xenopus laevis oocytes. These proteins are thought to function in the regulation of the membrane potential and in transepithelial ion absorption and secretion in the kidney. They possess two putative transmembrane a-helical spanners (TMSs) with cytoplasmic N- and C-termini and a large luminal loop that may be glycosylated. The bovine protein is 437 amino acyl residues in length and has the two putative TMSs at positions 223-239 and 367-385. The human nuclear protein is much smaller (241 residues). A C. elegans homologue is 260 residues long.
Sodium Bicarbonate Cotransporters
The novel human protein provided by the present invention is related to the family of sodium bicarbonate cotransporters, and shows a particularly high degree of similarity to the mouse sodium bicarbonate cotransporter isoform kNBC-3.
The sodium bicarbonate cotransporter provides the primary mechanism for transporting bicarbonate across the basolateral membrane in the kidney. At least three sodium bicarbonate cotransporter isoforms are located in the kidney and these may be functionally altered in various pathophysiologic states. For example, sodium bicarbonate cotransporter isoform 1 may be stimulated by metabolic acidosis, potassium depletion, and glucocorticoid excess and may be inhibited by bicarbonate loading or alkalosis. Sodium bicarbonate cotransporters are activated by cystic fibrosis transmembrane conductance regulator (CFTR) and play an important role in bicarbonate secretion in pancreatic duct cells. Furthermore, sodium bicarbonate cotransporters may play an important role in acid-base disorders such as proximal renal tubular acidosis (Soleimani et al., Kidney Int 2000 February;57(2):371-84).
The sodium-driven chloride/bicarbonate exchanger plays an important role in regulating intracellular pH in a wide variety cells by transporting extracellular sodium and bicarbonate into cells in exchange for intracellular chloride and H(+), thereby raising intracellular pH (Wang et al., J Biol Chem 2000 Nov. 10; 275(45):35486-90).
Transporter proteins, particularly members of the sodium bicarbonate cotransporter subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown transport proteins. The present invention advances the state of the art by providing previously unidentified human transport proteins.